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Chapter 17 - 1
ISSUES TO ADDRESS...
• Why does corrosion occur?
• What metals are most likely to corrode?
• How do temperature and environment affectcorrosion rate?
• How do we suppress corrosion?
CHAPTER 17:
CORROSION AND DEGRADATION
Chapter 17 - 2
• Corrosion:-- the destructive electrochemical attack of a material.-- Al Capone's
ship, Sapona,off the coastof Bimini.
• Cost:-- 4 to 5% of the Gross National Product (GNP)*-- this amounts to just over $400 billion/yr**
Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.
THE COST OF CORROSION
Leaking battery
Chapter 17 - 3
Electrochemical Considerations
• Oxidation (anodic reaction)
M Mn+ + ne-
metal atoms lose or give up electrons
• Reduction (Cathodic reaction)
Mn+ + ne- M
Mn+ + e- M(n-1)-
metal ions gain electrons
Chapter 17 - 4
• Two reactions are necessary:-- oxidation reaction:-- reduction reaction:
Zn → Zn2++ 2e−
2H++ 2e−
→ H2(gas)
• Other reduction reactions:-- in an acid solution -- in a neutral or base solution
O2 + 4H++ 4e−
→ 2H2O O2 + 2H2O + 4e−→ 4(OH)−
Adapted from Fig. 17.1, Callister 7e. (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)
CORROSION OF ZINC IN ACID
Zinc
Oxidation reactionZn Zn2+
2e-Acid solution
reduction reaction
H+H+
H2(gas)
H+
H+
H+
H+
H+
flow of e-
in the metal
Chapter 17 - 5
Not all metallic materials oxidize to form ions with the same degree of ease.
Chapter 17 - 6
Galvanic couple: two metals eclectically connected in a liquid electrolyte wherein one metal becomes an anode and corrodes, while the other acts as a cathode.
Chapter 17 - 7
The magnitude of such a voltage is the driving force for the electrochemical oxidation-reduction reaction.
Chapter 17 - 8
STANDARD HYDROGEN (EMF) TEST
• Two outcomes:
0ometal <V (relative to Pt)
Standard Electrode Potential
--Metal sample mass
--Metal is the cathode (+)
Mn+
ions
ne-
e- e-
25°C1M Mn+ sol’n 1M H+ sol’n
Pla
tinum
meta
l, M
H+
H+2e-
Adapted from Fig. 17.2, Callister 7e.
0ometal >V (relative to Pt)
--Metal sample mass
--Metal is the anode (-)
Pla
tinum
meta
l, M Mn+
ions
ne- H2(gas)
25°C1M Mn+ sol’n 1M H+ sol’n
2e-
e-e-
H+
H+
electromotive force
Chapter 17 - 9
STANDARD EMF SERIES• EMF series
AuCuPbSnNiCoCdFeCrZnAlMgNaK
+1.420 V+0.340- 0.126- 0.136- 0.250- 0.277- 0.403- 0.440- 0.744- 0.763- 1.662- 2.363- 2.714- 2.924
metal Vmetalo
Data based on Table 17.1, Callister 7e.
more
anod
icm
ore
cath
odic
metalo
• Metal with smallerV corrodes.
• Ex: Cd-Ni cell
∆V = 0.153V
o
Adapted from Fig. 17.2, Callister 7e.
-
1.0 M
Ni2+ solution
1.0 M
Cd2+ solution
+
25°C NiCd
Chapter 17 -10
CORROSION IN A GRAPEFRUIT
Cu Zn
Zn2+
2e- oxidationreduction
Acid
H+H+
H+
H+
H+
H+
H+
-+AnodeCathode
O2 + 4H++ 4e−
→ 2H2O
2H++ 2e−
→ H2(gas)
Chapter 17 -11
• Ex: Cd-Ni cell withstandard 1 M solutions
EFFECT OF SOLUTION
CONCENTRATION
1530oCd
oNi .VV =−
-
Ni
1.0 M
Ni2+ solution
1.0 M
Cd2+ solution
+
Cd 25°C
• Ex: Cd-Ni cell withnon-standard solutions
Y
Xln
nF
RTVVVV −−=−
oCd
oNiCdNi
n = #e-
per unitoxid/redreaction(= 2 here)F = Faraday'sconstant= 96,500C/mol.
• Reduce VNi - VCd by--increasing X--decreasing Y
- +
Ni
Y M
Ni2+ solution
X M
Cd2+ solution
Cd T
Chapter 17 -12
GALVANIC SERIES• Ranks the reactivity of metals/alloys in seawater
Based on Table 17.2, Callister
7e. (Source of Table 17.2 is M.G. Fontana, Corrosion
Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)
PlatinumGoldGraphiteTitaniumSilver316 Stainless SteelNickel (passive)CopperNickel (active)TinLead316 Stainless SteelIron/SteelAluminum AlloysCadmiumZincMagnesium
more
anod
ic(a
ctive)
more
cath
odic
(inert
)
Chapter 17 -13
• Uniform AttackOxidation & reductionoccur uniformly oversurface.
• Selective LeachingPreferred corrosion ofone element/constituent(e.g., Zn from brass (Cu-Zn)).
• Stress corrosionStress & corrosionwork togetherat crack tips.
• GalvanicDissimilar metals arephysically joined. Themore anodic onecorrodes.(see Table17.2) Zn & Mgvery anodic.
• Erosion-corrosionBreak down of passivatinglayer by erosion (pipeelbows).
FORMS OF CORROSION
Formsof
corrosion
• Crevice Between two
pieces of the same metal.
Fig. 17.15, Callister 7e. (Fig. 17.15 is courtesy LaQue Center for Corrosion Technology, Inc.)
Rivet holes
• IntergranularCorrosion alonggrain boundaries,often where specialphases exist.
Fig. 17.18, Callister 7e.
attacked zones
g.b. prec.
• PittingDownward propagationof small pits & holes.
Fig. 17.17, Callister 7e.(Fig. 17.17 from M.G.Fontana, Corrosion
Engineering, 3rd ed.,McGraw-Hill BookCompany, 1986.)
Chapter 17 -14
• Self-protecting metals!-- Metal ions combine with O
to form a thin, adhering oxide layer that slows corrosion.
• Reduce T (slows kinetics of oxidation and reduction)
• Add inhibitors-- Slow oxidation/reduction reactions by removing reactants
(e.g., remove O2 gas by reacting it w/an inhibitor).-- Slow oxidation reaction by attaching species to
the surface (e.g., paint it!).
CONTROLLING CORROSION
Metal (e.g., Al, stainless steel)
Metal oxide
Adapted from Fig. 17.22(a), Callister 7e. (Fig. 17.22(a) is from M.G. Fontana, Corrosion
Engineering, 3rd ed., McGraw-Hill Book Co., 1986.)
steel pipe
Mg anode
Cu wiree-
Earth
Mg2+
e.g., Mg Anode
• Cathodic (or sacrificial) protection-- Attach a more anodic material to the one to be protected.
Adapted from Fig. 17.23, Callister
7e. steel
zinczinc
Zn2+
2e- 2e-
e.g., zinc-coated nail
Chapter 17 -15
Chapter 17 -16
• Corrosion occurs due to:-- the natural tendency of metals to give up electrons.-- electrons are given up by an oxidation reaction.-- these electrons then used in a reduction reaction.
• Metals with a more negative Standard ElectrodePotential are more likely to corrode relative toother metals.
• The Galvanic Series ranks the reactivity of metals inseawater.
• Increasing T speeds up oxidation/reduction reactions.
• Corrosion may be controlled by:-- using metals which form
a protective oxide layer-- reducing T
-- adding inhibitors-- painting-- using cathodic protection.
SUMMARY